Geological-Geophysical map series of Chinas Land and Sea on the scale of 1:5 000 000 include eight geological and geophysical maps: Free-air gravity anomaly map, Bouguer gravity anomaly map, Magnetic gravity map, Moho depth map, Seismic Tomography Maps, Geological map, Tectonic frame map, and Tectonic evolution maps. The mapping area includes Chinese mainland and China seas and the adjacent land and sea area. This paper discusses the principal and method for choosing proper projection, which is the key question and mathematical base of GIS mapping. Based on the research of the former similar mapping and the understanding of the nature of different projections, we choose two common used equal-angle projections, i.e. Mercator and Lambert conformal conic projections for consideration. The deformation ratios of length and area are calculated under these two projections in the mapping area. The position and shape of Chinese land and sea boundaries are compared for testing different design solution of projection parameters. Lambert conformal conic projection is chosen finally not only because it causes less deformation, but also because it could limit the mapping area on a smaller scale which keep the map subdivision to a minimum. The central longitude of Lambert conformal conic projection is set to 105°E and the two standard parallels are 15°N and 40°N. The data requirement of different maps is also taken into account when setting the four corner points of the map. With this solution, Chinese mainland has a good visual effect, geological elements have small shape and direction deformation, and length and area distortion ratio can be controlled within 10%.